Matty,
I'm afraid you can't have your cake and eat it too.
If you apply polish to a 2000 grit surfaced ball, you are going to get more length, no matter what you do. That length is going to reduce the midlane reading ability of the ball. Period. If you hand apply polish lightly, the reduction can be minimal. If you apply a lot with a spinner, the reduction can be a lot.
CobaltBomb had a good suggestion in sanding the ball to 4000 grit. That will add length without a large decrease in midlane read, just a small amount, relatively speaking.
HOWEVER ....
having read your post on your 505T here and on BowlingChat, you said your Tropical Heat is reacting much better than your 505T AND your Marvel. That tells me everything and in the future it should tell you the same also. Both these ball are stronger oil readers than the TH. That is, they need mmore oil and will react earlier and stronger than the TH to oil. On the other hand it also means they will burn up all their energy in the heads and the midlane when you try to use them on too little oil and the TH will be the ball to use, in that case.
I think you should save the Marvel and the 505T for when you have more oil to allow them to conserve their energy and use the TH when you see the lesser amount of oil you're seeing now.
Good luck!
"None are so blind as those who will not see." "Some bowlers are crazy as pickled cats."